U.S. patent number 5,423,899 [Application Number 08/093,087] was granted by the patent office on 1995-06-13 for dispersion alloyed hard metal composites and method for producing same.
This patent grant is currently assigned to Newcomer Products, Inc.. Invention is credited to Jack Krall, Anders Olsson.
United States Patent |
5,423,899 |
Krall , et al. |
June 13, 1995 |
Dispersion alloyed hard metal composites and method for producing
same
Abstract
A method for forming a sintered hard metal composite is provided
in which unsintered nodules of a pre-blended hard metal powder of a
first grade are uniformly dispersed into unsintered nodules of a
pre-blended hard metal composite of a second grade. The pre-blended
hard metal powders form a composite powder blend which is
subsequently pressed and sintered to form the dispersion alloyed
hard metal composite. A sufficient amount of pressing lubricant is
provided to one of the pre-blended hard metal powders so that each
of the hard metal powders shrinks at approximately the same rate
relative to the application of pressure during the compacting
process. The pressing lubricant is added to that hard metal powder
which shrinks more during sintering. By providing uniform shrinkage
of the constituent powder grades, migration of the binder from one
constituent grade to the other constituent grade is minimized,
thereby allowing the composite powder blend to achieve superior
properties because each of the constituent grades maintains its own
integrity. In addition, the uniform shrinkage of the constituent
powders prevents irregular surface conditions of the "as sintered"
surface.
Inventors: |
Krall; Jack (Latrobe, PA),
Olsson; Anders (Fagersta, SE) |
Assignee: |
Newcomer Products, Inc.
(Latrobe, PA)
|
Family
ID: |
22236967 |
Appl.
No.: |
08/093,087 |
Filed: |
July 16, 1993 |
Current U.S.
Class: |
75/231; 75/240;
75/242; 419/36; 419/38 |
Current CPC
Class: |
B22F
1/0003 (20130101); B22F 1/0059 (20130101); B22F
3/156 (20130101); B22F 2003/023 (20130101); B22F
2998/00 (20130101); B22F 2999/00 (20130101); B22F
2998/00 (20130101); B22F 3/156 (20130101); B22F
2999/00 (20130101); B22F 2203/05 (20130101); B22F
1/0003 (20130101) |
Current International
Class: |
B22F
1/00 (20060101); C22C 029/12 (); B22F 003/12 () |
Field of
Search: |
;75/231,240,242,246
;419/15,18,36,38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0095668 |
|
Dec 1983 |
|
EP |
|
1525290 |
|
Jan 1981 |
|
GB |
|
1588208 |
|
Apr 1981 |
|
GB |
|
2074609A |
|
Mar 1983 |
|
GB |
|
WO90/11855 |
|
Oct 1890 |
|
WO |
|
Other References
Article "Structure and Properties Of Dual Properties Carbide For
Rock Drilling" by Aronsson, Hartzell and Akerman. .
DIN 30 900 (Ed. Jul. 1982) "Terminologie der Pulvermetallurgie", p.
14 (No translation). .
F. Eisenkolb "Fortschritte der Pulvermetallurgie", vol. II,
Akademie-Vlg. Berlin, 1963, pp. 493-543 No translation..
|
Primary Examiner: Mai; Ngoclan
Attorney, Agent or Firm: Buchanan Ingersoll Dever; Michael
L.
Claims
We claim:
1. A method for forming a sintered hard metal composite comprising
the steps of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a first grade into unsintered nodules of a
pre-blended hard metal powder of a second different grade to form a
composite powder blend;
(b) processing said pre-blended hard metal powder of a first grade
and said pre-blended hard metal powder of a second grade such that
each said hard metal powder shrinks by approximately the same rate
when compacted and sintered;
(c) pressing said composite powder blend; and
(d) sintering said composite powder blend.
2. A method for forming a sintered hard metal composite comprising
the steps of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a first grade into unsintered nodules of a
pre-blended hard metal powder of a second different grade to form a
composite powder blend;
(b) determining the shrinkage of said pre-blended hard metal powder
of a first grade and said pre-blended hard metal powder of a second
grade relative to the application of pressure;
(c) providing a sufficient amount of a pressing lubricant to the
hard metal powder which shrinks more such that each said hard metal
powder shrinks at approximately the same rate relative to the
application of compacting pressure;
(d) pressing said composite powder blend; and
(e) sintering said composite powder blend.
3. The method of claim 2 wherein said each of said pre-blended hard
metal powder of a first grade and said pre-blended hard metal
powder of a second grade comprise tungsten carbide and a binder,
said pressing lubricant providing sufficient shrinkage adjustment
of said at least one of said pre-blended hard metal powder of a
first grade and said pre-blended hard metal powder of a second
grade to prevent migration of said binder from which of said
pre-blended hard metal powder of a first grade and said pre-blended
hard metal powder of a second grade that shrinks less relative to
the application of pressure.
4. The method of claim 3 wherein said binder is cobalt.
5. The method of claim 3 wherein said binder is nickel.
6. The method of claim 3 wherein said binder is iron.
7. A method for forming a sintered hard metal composite comprising
the steps of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a first grade into unsintered nodules of a
pre-blended hard metal powder of a second different grade to form a
composite powder blend
(b) providing a sufficient amount of a stearate compound pressing
lubricant to at least one of said pre-blended hard metal powder of
a first grade and said pre-blended hard metal powder of a second
grade such that each said hard metal powder shrinks at
approximately the same rate relative to the application of
compacting pressure;
(c) pressing said composite powder blend; and
(d) sintering said composite powder blend.
8. The method of claim 7 wherein said pressing lubricant is stearic
acid.
9. A sintered hard metal composite comprising unsintered nodules of
a pre-blended hard metal powder of a first grade uniformly
dispersed among unsintered nodules of a pre-blended hard metal
powder of second grade, said pre-blended hard metal powder of a
first grade and said pre-blended hard metal powder of a second
grade having distinctively different properties from each other,
wherein the integrity of the constituent grades is maintained after
sintering, and the resulting composite exhibits hardness and
toughness properties of the hard metal powder of the first grade
and the hard metal powder of the second grade, wherein at least one
of said pre-blended hard metal powder of a first grade and said
pre-blended hard metal powder of a second grade is processed such
that each said hard metal powder shrinks by approximately the same
rate when compacted and sintered.
10. A sintered hard metal composite comprising unsintered nodules
of a pre-blended hard metal powder of a first grade uniformly
dispersed among unsintered nodules of a pre-blended hard metal
powder of second grade, said pre-blended hard metal powder of a
first grade and said pre-blended hard metal powder of a second
grade having distinctively different properties from each other,
wherein the integrity of the constituent grades is maintained after
sintering, and the resulting composite exhibits hardness and
toughness properties of the hard metal powder of the first grade
and the hard metal powder of the second grade, wherein a stearate
compound pressing lubricant is provided in at least one of said
pre-blended hard metal powder of a first grade and said pre-blended
hard metal powder of a second grade.
11. The composite of claim 10 wherein said pressing lubricant is
stearic acid.
12. A sintered hard metal composite comprising unsintered nodules
of a pre-blended hard metal powder of a first grade uniformly
dispersed among unsintered nodules of a pre-blended hard metal
powder of second grade, said pre-blended hard metal powder of a
first grade and said pre-blended hard metal powder of a second
grade having distinctively different properties from each other,
wherein the integrity of the constituent grades is maintained after
sintering, and the resulting composite exhibits hardness and
toughness properties of the hard metal powder of the first grade
and the hard metal powder of the second grade, wherein a pressing
lubricant is provided in at least one of said pre-blended hard
metal powder of a first grade and said pre-blended hard metal
powder of a second grade, wherein said each of said hard metal
powder of a first grade and said hard metal powder of a second
grade comprise tungsten carbide and a binder, said pressing
lubricant providing sufficient shrinkage adjustment of said at
least one of said pre-blended hard metal powder of a first grade
and said pre-blended hard metal powder of a second grade to prevent
migration of said binder from which of said pre-blended hard metal
powder of a first grade and said pre-blended hard metal powder of a
second grade that shrinks less relative to the application of
pressure.
13. The composite of claim 12 wherein said binder is cobalt.
14. A sintered hard metal composite comprising unsintered nodules
of a pre-blended hard metal powder of a first grade having a binder
provided therein uniformly dispersed among unsintered nodules of a
pre-blended hard metal powder of a second different grade having a
binder provided therein, wherein a sufficient amount of pressing
lubricant is provided in at least one of said pre-blended hard
metal powder of a first grade and said pre-blended hard metal
powder of a second grade to control the binder migration between
said pre-blended hard metal powder of a first grade and said
pre-blended hard metal powder of a second grade resulting from the
shrinkage difference between said pre-blended hard metal powder of
a first grade and said pre-blended hard metal powder of a second
grade during sintering.
15. The hard metal composite of claim 14 wherein sufficient
lubricant is added to equalize the shrinkage difference between
said pre-blended hard metal powder of a first grade and said
pre-blended hard metal powder of a second grade.
16. A method for forming a sintered hard metal composite having
predetermined composition and properties comprising the steps
of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a first grade having a binder provided therein into
unsintered nodules of a pre-blended hard metal powder of a second
different grade having a binder provided therein to form a
composite powder blend;
(b) providing a sufficient amount of pressing lubricant to at least
one of said pre-blended hard metal powder of a first grade and said
pre-blended hard metal powder of a second grade to control the
binder migration between said hard metal powders resulting from the
shrinkage difference between said hard metal powders during
sintering;
(c) pressing said composite powder blend; and
(d) sintering said composite powder blend.
17. A method for forming a sintered hard metal composite having
predetermined composition and properties comprising the steps
of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a submicron grain size grade having a binder
provided therein into unsintered nodules of a pre-blended hard
metal powder of a coarser grain size grade having a binder provided
therein to form a composite powder blend;
(b) providing a sufficient amount of pressing lubricant to at least
one of said pre-blended hard metal powder of a submicron grain size
grade and said pre-blended hard metal powder of a coarser grain
size grade to control the binder migration between said hard metal
powders resulting from the shrinkage difference between said hard
metal powders during sintering;
(c) pressing said composite powder blend; and
(d) sintering said composite powder blend,
wherein binder migration is governed by the equation:
wherein:
C(P).sub.o =weight percent of binder content in said pre-blended
hard metal powder of a submicron grain size grade in initial
powder;
C(M).sub.o =weight percent binder content in said pre-blended hard
metal powder of a coarser grain size grade in initial powder
P=percent of said pre-blended hard metal powder of a submicron
grain size grade by weight;
SHRINKD=shrinkage difference between said pre-blended hard metal
powder of a submicron grain size grade and said pre -blended hard
metal powder of a coarser grain size grade; and
A and B=constants, the value of which depends on the binder
material.
18. The method of claim 17 wherein said binder is cobalt and
constant A has the value -1.096 and constant B has the value
0.461.
19. The method of claim 16 wherein more than two hard metal
constituents are used in said pre-blended hard metal powders.
20. A method for forming a sintered hard metal composite having
predetermined composition and properties comprising the steps
of:
(a) uniformly dispersing unsintered nodules of a pre-blended hard
metal powder of a submicron grain size grade having a binder
provided therein into unsintered nodules of a pre-blended hard
metal powder of a coarser grain size grade having a binder provided
therein to form a composite powder blend;
b) providing a sufficient amount of pressing lubricant to at least
one of said pre-blended hard metal powder of a submicron grain size
grade and said pre-blended hard metal powder of a coarser grain
size grade to control the binder migration between said hard metal
powders resulting from the shrinkage difference between said hard
metal powders during sintering;
c) pressing said composite powder blend; and
(d) sintering said composite powder blend, wherein more than one
binder is used.
21. The method of claim 20 wherein one of the binders used is
cobalt.
22. The method of claim 20 wherein one of the binders is
nickel.
23. The method of claim 20 wherein one of the binders is iron.
Description
FIELD OF THE INVENTION
The present invention relates to hard metal composites and more
particularly to cemented carbide compositions having improved
properties and a method for their formation.
DESCRIPTION OF THE PRIOR ART
Hard metals are composites consisting of metal carbides, primarily
tungsten carbide, and a binder material, generally cobalt, and are
commonly known as cemented carbides. The metal carbide and binder
material are blended together as powders, pressed, and sintered in
a protective atmosphere or vacuum. During sintering, the binder
material, which may range from 1% to 25% by weight of the compact,
or higher, forms a liquid phase and completely surrounds the metal
carbide particles, thereby achieving full density. A "fully" dense
hard metal is generally considered one in which the actual density
is greater than 99.5% of the theoretical density of the
composite.
The resultant cemented tungsten carbide composite exhibits very
high hardness and relatively high toughness. Such composites are
widely used as metal cutting tools and mining or earth drilling
tools. In addition, these composites are used in metal stamping,
forming and powder compacting applications.
It is well known that the two most important factors affecting the
hardness and toughness properties of fully dense hard metal
composites are the binder content and the particle size (grain
size) of the metal carbides employed. The higher the binder content
of a composite, the lower the hardness. Conversely, the lower the
binder content of the composite, the lower its toughness. In
addition, the hardness of the composite increases as the particle
size of the metal carbide employed is decreased. To a lesser
extent, the toughness of a composite decreases as the particle size
of the metal carbide employed is decreased. Consequently, until
recently, it had always been necessary to sacrifice either the
hardness or toughness of the composite in order to improve the
other property by these means.
Recently, a new hard metal composite has been formed from a mixture
of two or more pre-blended, unsintered hard metal composites in
which the properties of each constituent composite are different.
Such a dispersion alloyed hard metal composite is discussed in U.S.
Pat. No. 4,956,012. Therein, the constituent components of the hard
metal composite are selected so that they have different grain
sizes, different binder contents, different metal carbide or
binders, or some combination of these. Primarily, the constituents
are chosen on the basis of their properties and compatibility, and
are chosen to utilize the superior properties of one of the
constituents without detrimentally affecting the desirable
properties of the other. As an example, a pro-blended composite
having superior hardness may be dispersed in a second composite
having superior toughness with the resultant material having a
hardness which approaches that of the harder constituent yet
maintains the toughness of the matrix constituent.
Although the hard metal composite disclosed in U.S. Pat. No.
4,956,012 produces a superior composite, it has been found that the
binder sometimes may tend to migrate during liquid phase sintering
in such a way that the physical properties of each constituent
component change and become more similar to each other. When this
occurs, the resulting hard metal composite performs more like a
traditional single mixture instead of utilizing the superior
properties of each of the constituents.
The amount of binder migration that occurs in traditional wafer or
gradient composites is affected by the temperature and duration of
sintering. It has been found that binder migration can be minimized
by sintering at extremely low temperatures. However, composites
manufactured in such a manner often do not reach full density and
have deficient structures and physical properties that differ from
those of the original design Consequently, there is a need for an
improved method for forming a hard metal composite which minimizes
the deleterious effects of binder migration.
SUMMARY OF THE INVENTION
In the present invention, it has been discovered that binder
migration sometimes occurs in the dispersion alloyed hard metal
composite as described in U.S. Pat. No. 4,956,012. Binder migration
occurs primarily when the composite is shrinking during sintering.
Equilibrium is reached when the composite reaches full density.
This differs from the traditional wafer composite wherein migration
of the binder continues after full density is reached until the
capillary forces are in equilibrium.
In a traditional wafer composite, when sintering different grades
of carbide together, the binder migrates from one material to the
other when it becomes hot enough to liquefy. Small, fine grains of
tungsten carbide have a much larger surface area to cover with the
binder relative to coarser grain carbides. As a result, the layers
of binder which bond to the fine grain carbides are very thin
whereas the layers of cobalt which bind the coarser grain carbides
are relatively thick provided the percentage of the binder is the
same for each composition.
Capillary forces are higher when the layers of the binder material
are very thin, causing the binder to be drawn or migrate from the
coarser grain carbides to the fine grain carbides. During the
liquid phase, the migration continues in a traditional wafer
composite until the thickness of both binder layers of the
composite are equal. That is to say, migration continues until the
capillary forces between the two materials reaches an
equilibrium.
If the wafer composite is cooled until the binder is no longer
liquid, migration stops. Heating the part again causes migration to
pick-up where it left off. If the sintering temperature is
increased, the surface tension and viscosity of the binder
decreases, allowing the binder to migrate at a faster rate until
equilibrium is reached. When equilibrium is attained, the
properties of the separate composites are similar, thereby
minimizing the value of having a composite.
When pressed powders are sintered, a substantial amount of
shrinkage takes place until the constituent composites are fully
dense. Each grade of carbide shrinks at a different rate. If a
wafer composite is sintered, one of the constituent layers will
shrink more than the other, causing the piece to distort or warp.
When the binder migrates from one layer to another, there is a
volume change which also contributes to warpage.
In a composite carbide formed according to U.S. Pat. No. 4,956,012,
where the constituents have the same cobalt content before
sintering, the binder migrates from the material that shrinks the
least to the material that shrinks the most. Although the shrinkage
of the materials is not the only factor affecting binder migration,
it is the major parameter to control the final cobalt content of
the constituents. In general, most fine grain carbide grades shrink
more than coarse grain carbide grades during sintering. In this
respect, the direction of binder migration in such a composite is
the same as for a wafer composite, but the mechanism or driving
force is different. Only minimal migration occurs in a pellet
composite formed in accordance with U.S. Pat. No. 4,956,012 when
the shrinkages of the two constituents are equal regardless of
their grain size.
The shrinkage of the constituent components of the alloyed hard
metal composite can be modified by means of a pressing lubricant.
The pressing lubricant can be used to adjust the shrinkage of each
constituent material until such shrinkage is equal. When such
shrinkage is equal, binder migration will be nearly eliminated.
When pressure is applied to powder metals, those metals are
compacted to a "green density". When a lubricant such as stearic
acid or an ethomeen compound is added to the powder metal, the
resistance of the metal to compaction is reduced. As a result, when
a lubricant is added the part compacts further, producing a greater
"green density" wherein the percentage of additional shrinkage that
occurs during final sintering is reduced. By adjusting the type and
quantities of lubricants, it is possible to control the shrinkage
of each composite component.
By controlling the binder migration, the physical properties of the
composite are also controlled. The tough matrix maintains its
optimal strength while the pellet maintains its hardness and wear
resistance. As a result, the properties of each component of a
pellet composite are significantly enhanced over those of a wafer
composite made of the same materials.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photomicrograph showing a magnification at 1500
diameters of a submicron grained hard metal whose tungsten carbide
grains average less than 1 micron.
FIG. 2 is a photomicrograph showing a magnification at 1500
diameters of a medium grained hard metal whose tungsten carbide
grains range from 3 to 5 microns.
FIG. 3 is a photomicrograph showing a magnification at 150
diameters of a dispersion alloyed hard metal composite according to
the present invention.
FIG. 4 is a photomicrograph showing a magnification at 1500
diameters of the dispersion alloyed hard metal composite of FIG. 3
showing the interface between the medium grained hard metal and the
submicron grained hard metal constituents.
FIG. 5 is a graph showing the shrinkage rates of the hard metal
powder composite of FIG. 1, hard metal powder composite of FIG. 2,
and a hard metal powder composite of FIG. 3 modified in accordance
with the present invention.
FIG. 6 is a graph comparing the predicted cobalt migration to the
observed cobalt migration for a dispersion alloyed hard metal
composite formed in accordance with the present invention.
FIG. 7 is a graph-showing the shrinkage rates of a coarse grained
hard metal powder matrix, a submicron grained hard metal powder
pellet having no lubricant added, and a submicron grained hard
metal powder pellet having added lubricant.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the microstructure of a sintered submicron grained
hard metal composed of tungsten carbide and a cobalt binder. The
particle size of the tungsten carbide is generally less than one
micron, although a few grains are in excess of one micron. The
binder content of this submicron grained hard metal is 6% by
weight. This submicron grained hard metal is a grade used for high
wear resistance application where little impact resistance is
required. An example of such a hard metal is Newcomer Products,
Inc. Grade NP32 having 6% cobalt and the balance being submicron
tungsten carbide.
FIG. 2 shows the microstructure of a sintered medium grained hard
metal composed of tungsten carbide particles surrounded by a cobalt
binder. The particle size of the tungsten carbide generally ranges
from 3 to 5 microns. The binder content of this medium grained hard
metal is 6% by weight. This medium grained hard metal is a typical
grade for high impact resistance application. An example of such a
hard metal is Newcomer Products, Inc. Grade N406 having 6% cobalt
and the balance being 3 to 5 micron diameter tungsten carbide.
The submicron grained hard metal of FIG. 1 is a "hard" composition.
The medium grained hard metal of FIG. 2 is a "tough" composition.
In the present invention, the "tough" composite and the "hard"
composite are combined to form a dispersion alloyed hard metal
composite having the toughness of the "tough" composite and wear
resistance nearly that of the "hard" composite.
The dispersion alloyed hard metal composite of the present
invention is formed by dispersing unsintered nodules of the "hard"
composite of FIG. 1 in unsintered nodules of the "tough" composite
of FIG. 2. The constituents of the dispersion alloyed hard metal
composite are dispersed prior to pressing and sintering of the
constituent composites. The dispersion alloyed hard metal composite
may contain up to approximately 50% by weight of the "hard"
constituent and the balance as the "tough" matrix constituent.
Any pelletizing process can be used to produce the pellets or
nodules of the select grade. Preferred processes include vibratory
pelletizing, wet pelletizing, slugging and granulating methods, and
spray drying. The "hard" and "tough" components are then precisely
weighed and mixed by a very gentle dry-mixing of the pre-blended
pellets to avoid breaking the pellets. Pressing and sintering of
the hard metal composite is then performed by normal means.
Secondary sintering processes, such as hot isostatic pressing or a
low pressure sinter-hip process, may be performed to enhance the
resultant properties of the hard metal composite.
FIG. 3 shows the dispersion of the "hard" constituent (Grade NP32)
and the "tough" constituent (Grade N406) at 150 magnifications in
the sintered state. Nodules of the submicron grained composite are
seen as islands dispersed through the lighter-colored medium
grained matrix. FIG. 4 shows the dispersion alloyed hard metal
composite of FIG. 3 at 1500 magnification. The sintering is
complete within the individual constituents and between the
differing constituent grades. This provides a fully dense
composite. Full density is achieved because the pressing and
sintering of the constituent composites does not occur until they
are fully mixed. It has been found that the medium grained hard
metal shown in FIG. 2 shrinks less than the submicron grained hard
metal of FIG. 1. The submicron grained metal powder shrinks to a
greater degree than the medium grained hard metal powder. Thus,
with respect to the composite shown in FIG. 3, if the "hard"
pellets have a greater shrinkage than the "tough" matrix, the
volume reduction during sintering will be greater for the dispersed
pellets. This can cause portions of the dispersed pellets to
separate from the matrix resulting in voids within the composite.
While sinter-hipping or secondary hipping operations can correct
most of these defects, the net result is usually a composite with
inferior properties or with added costs to manufacture.
An example of a dispersion alloyed hard metal composite is Newcomer
Products, Inc. grade NJL35 having 65% N406 grade carbide as the
"matrix grade" and 35% NP32 grade carbide as the dispersed pellets.
The physical properties of NJL35, N406 and NP32 are presented in
Table I below:
TABLE I ______________________________________ Physical Properties
NJL35 N406 NP32 ______________________________________ Density
15.00 15.00 14.95 Hardness 91.5 90.7 92.7 TRS 425,000 425,000
380,000 HC 225 160 280 Porosity A02 A02 A02
______________________________________
The component that shrinks the most will appear as an indent or
recessed pit on the surface of the as-sintered composite. This
rough surface is detrimental to the performance of cutting tools as
well as wear parts. Particularly, this rough surface is detrimental
when impacts and internal stresses are involved. Secondary grinding
operations can produce smooth surfaces, but this extra operation is
not always practical or cost effective. If the constituent hard
metal powders are designed to shrink at the same rate, the
deleterious effects of different shrinking rates are eliminated as
are the problems of binder migration. In order to provide equal
shrinkage, a lubricant is added to the "hard" powder to cause it to
shrink less than the original powder when equal compacting
pressures are applied. This lubricant, which preferably is a
stearate compound such as stearic acid, is added in a heptane
solvent to the binder prior to pelletizing the constituent hard
metal powder. By adding the lubricant, the shrinkage of the
constituent parts is made uniform, thereby preventing the volume
reduction effects during sintering and eliminating binder
migration. This stearate lubricant can be added to the composite in
place of or in addition to the paraffin normally added to the
powders for pelletizing and compacting.
FIG. 5 shows a shrinkage comparison of a "hard" tungsten carbide
grade designated NP32 and a "tough" tungsten carbide grade
designated N406. When stearic acid is added to the "hard" NP32
grade, its shrinkage rate is adjusted to approximate the shrinkage
of the "tough" N406 composite. As to the example in FIG. 5, the
dispersion alloyed hard metal composite formed from N406 and NP32
plus stearic acid constituents should be pressed or compacted at
approximately 25-30 tons per square inch pressure for the least
amount of cobalt migration to occur during sintering. This
compacting pressure is found by the intersection of the shrinkage
curves for the N406 constituent and the NP32 with added stearic
acid constituent, although minimal binder migration would occur at
any pressure because the shrinkage is similar over the entire curve
compared to the submicron grade without special lubricants
added.
It has been found that temperature has little or no effect on the
amount of binder migration that occurs in the dispersion alloyed
hard metal composite formed in accordance with this invention.
Likewise, sintering time has little or no effect on the amount of
binder migration. Once full density of the resulting composite is
reached, binder migration ceases. Because the shrinkages of the
"tough" and "hard" constituents are equalized, the volume reduction
during sintering of the composite is equal in all directions. This
results in a composite that maintains its shape throughout
sintering and maintains a smooth "as sintered" surface
condition.
In order to show that the mechanism for binder migration differs
between the dispersed composite of the present invention and
traditional composites, we made a traditional wafer composite
having the same materials as the dispersed composite of FIG. 3. In
order to compare the results with the dispersed composite having
35% by weight pellets, the wafer composite was formulated such that
one of the wafer layers was 35% by weight of the wafer
composite.
In conformance with traditional technology, the wafer composite
exhibited cobalt migration that continued each time the composite
was heated or reheated until the capillary forces reached
equilibrium. Although the dispersed pellet without added lubricant
exhibited cobalt migration, the amount of such migration was never
as great as in the wafer composite. Moreover, the dispersed
composite did not exhibit additional cobalt migration upon
reheating. Furthermore, adding lubricants to the dispersed
composite to equalize shrinkage resulted in nearly zero migration.
These differences in results between the wafer composite and the
dispersed composite show that different mechanisms are involved in
the binder migration.
We have developed an equation to predict the amount of cobalt
migration based on the shrinkage difference of the components and
the initial cobalt content of the constituents. That equation is
represented below where the amount of cobalt migration is expressed
as the change of cobalt content of the hard constituent:
where:
Co(P)=cobalt in pellets in sintered composite
Co(M).sub.o =initial cobalt in matrix
Co(P).sub.o =initial cobalt in pellets
P=percent of pellets by weight
SHRINKD=shrinkage difference between pellets and matrix
A and B=constants derived from FIG. 6
Equation (2) is based on two parts. The first term is derived from
the assumption that the constituents want to equalize the cobalt
content and the second term is the influence of a difference in
shrinkage.
Fifteen different mixtures have been made and analyzed regarding
cobalt migration. Data derived from the analysis is set forth in
FIG. 6. Using statistical methods, the constants "A" and "B" have
been determined from the accumulated data resulting in FIG. 6. The
values of "A" and "B" used to fit the data in FIG. 6 are -1.096287
and 0.46081, respectively. These values of "A" and "B" are each
statistically significant to more than 95%.
From Equation (2), it can be seen that when Co(P).sub.o equals
Co(M).sub.o, the only significant factor in binder migration is the
shrinkage difference. Consequently, when the shrinkage difference
is reduced to 0, binder migration does not occur to any significant
amount. When Co(P).sub.o does not equal Co(M).sub.o, binder
migration can still be reduced to nearly zero by altering the
shrinkage difference enough to counteract the natural tendency for
the cobalt to migrate.
Equation (2) above can also be used to calculate the desired
compositions of the starting components and the shrinkage
difference needed to create a sintered composite that has had a
controlled intentional amount of binder migration in order to
formulate desired compositions and properties.
It has been found that the above equation can be used for
composites having more than two components and for composites
having different binders such as cobalt, nickel, iron or
combinations of binders. The equation also can be used when
different mutually soluble binders are used for each component.
FIG. 7 shows the relationship of compacting tooling requirements
and design to the desired shrinkage adjustment. In the example
shown in FIG. 7, both the matrix and the pellet have 6% cobalt
content. According to the equation, the shrinkage difference must
be zero in order to produce a composite having no cobalt
migration.
The shrinkage curve produced for the pellet having no added
lubricant does not intersect the shrinkage curve produced for the
matrix. Consequently, there is no point at which there is a zero
shrinkage difference between the pellet and the matrix.
In contrast, the shrinkage curve for the pellet having added
lubricant does intersect the matrix shrinkage curve at 16.5%
shrinkage and a compacting pressure of 10 tons per square inch.
Accordingly, compacting tooling designed using these parameters and
using these components will produce a sintered composite having
nearly zero binder migration.
In the above description, tungsten carbide was used as a
representative hard metal and cobalt was used as a representative
binder for the hard metal composite. It should be understood that
the present invention applies equally as well to other hard metals
such as titanium carbide, tantalum carbide, niobium carbide, and
combinations of these carbides and combinations of these carbides
with tungsten carbide. It should also be understood that the
present invention applies equally as well to other binders such as
iron, nickel and other materials that form a liquid state during
sintering as well as mixtures thereof.
In the foregoing specification certain preferred practices and
embodiments of this invention have been set out, however, it will
be understood that the invention may be otherwise embodied within
the scope of the following claims.
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